Lag spikes in Windows and in-game

Hi all,
So after owning a computer that has run all games on high settings no problem for the past year, i've hit a problem by the name of lag spikes. First off , here are my specs

AMD Phenom X4 955 (3.2gHz)
HYNIX 4GB 1333MHz DDR3 Ram
AMD XFX Radeon HD5770 1Gb
ASUS M4N68T-M motherboard
Windows 7 64bit

Alright , so the issue started about two months ago, and this is what is happening:

*Every 5 minutes or so in Windows, I get lag. The computer doesn't seem to neccesarily slow down, but when i hover over the preview window on the taskbar, the preview window will "lag" up
*Also in firefox, when I create a new tab and it slides across, instead of a smoth motion like it should be, it lags across.

the lag seems to happen in all programs , but this is just where i noticed it first!

Now, in game:

*In Team Fortress 2, every 5 minutes or so (the same lag im assuming that affects windows) occurs, and my fps drops from a solid 100+, to about 20 or less. I have a fps meter ingame , which seems to say the fps is higher than it really is

*In Call of Duty: Black Ops, every 5 minutes or so, i get major fps lag also, similar to tf2 but worse, because not just the fps is affected , but also the sound, which gets stuttery.

I first assumed my graphics card was at fault, and too that into the shop where the guy tested it and said there was no problem.

Does anyone have any idea of what could be at fault? Thanks in advance  

EDIT: Cpu is running at 73 deg celcius